Chapter 16 Using Authentication Services (Tasks)

This chapter provides information about how to use Secure RPC to authenticate a host and a user across an NFS mount. The following is a list of the topics in this chapter.

• Overview of Secure RPC

• Administering Secure RPC (Task Map)

Overview of Secure RPC

Secure RPC (Remote Procedure Call) protects remote procedures with an authentication mechanism. The Diffie-Hellman authentication mechanism authenticates both the host and the user who is making a request for a service. The authentication mechanism uses Data Encryption Standard (DES) encryption. Applications that use Secure RPC include NFS and the name services, NIS and NIS+.

NFS Services and Secure RPC

NFS enables several hosts to share files over the network. Under the NFS service, a server holds the data and resources for several clients. The clients have access to the file systems that the server shares with the clients. Users who are logged in to the client systems can access the file systems by mounting the file systems from the server. To the user on the client system, it appears as if the files are local to the client. One of the most common uses of NFS allows systems to be installed in offices, while storing all user files in a central location. Some features of the NFS service, such as the -nosuid option to the mount command, can be used to prohibit the opening of devices and file systems by unauthorized users.

The NFS service uses Secure RPC to authenticate users who make requests over the network. This process is known as Secure NFS. The Diffie-Hellman authentication mechanism, AUTH_DH, uses DES encryption to ensure authorized access. The AUTH_DH mechanism has also been called AUTH_DES. For more information, see the following:

• To set up and administer Secure NFS, see Administering the Secure NFS System in System Administration Guide: Network Services.

• To set up the NIS+ tables and enter names in the cred table, see System Administration Guide: Naming and Directory Services (NIS+).

• For an outline of the transactions that are involved in RPC authentication, see Implementation of Diffie-Hellman Authentication.

DES Encryption With Secure NFS

The Data Encryption Standard (DES) encryption functions use a 56-bit key to encrypt data. If two credential users or principals know the same DES key, they can communicate in private by using the key to encipher and decipher text. DES is a relatively fast encryption mechanism. A DES chip makes the encryption even faster. However, if the chip is not present, a software implementation is substituted.

The risk of using just the DES key is that an intruder can collect enough cipher-text messages that were encrypted with the same key to be able to discover the key and decipher the messages. For this reason, security systems such as Secure NFS need to change the keys frequently.

Kerberos Authentication

Kerberos is an authentication system that was developed at MIT. Some encryption in Kerberos is based on DES. Kerberos V4 support is no longer supplied as part of Secure RPC. However, a client-side and server-side implementation of Kerberos V5, which uses RPCSEC_GSS, is included with this release. For more information, see Chapter 21, Introduction to the Kerberos Service.

Diffie-Hellman Authentication and Secure RPC

The Diffie-Hellman (DH) method of authenticating a user is nontrivial for an intruder to crack. The client and the server have their own private key, which they use with the public key to devise a common key. The private key is also known as the secret key. The client and the server use the common key to communicate with each other. The common key is encrypted with an agreed-upon encryption function, such as DES.

Authentication is based on the ability of the sending system to use the common key to encrypt the current time. Then, the receiving system can decrypt and check against its current time. The time on the client and the server must be synchronized. For more information, see Managing Network Time Protocol (Tasks) in System Administration Guide: Network Services.

The public keys and private keys are stored in an NIS or NIS+ database. NIS stores the keys in the publickey map. NIS+ stores the keys in the cred table. These files contain the public key and the private key for all potential users.

The system administrator is responsible for setting up NIS maps or NIS+ tables, and for generating a public key and a private key for each user. The private key is stored in encrypted form with the user's password. This process makes the private key known only to the user.

Implementation of Diffie-Hellman Authentication

This section describes the series of transactions in a client-server session that use Diffie-Hellman authentication (AUTH_DH).

Generating the Public Keys and Secret Keys for Secure RPC

Sometime prior to a transaction, the administrator runs either the newkey or the nisaddcred command to generate a public key and a secret key. Each user has a unique public key and secret key. The public key is stored in a public database. The secret key is stored in encrypted form in the same database. The chkey command changes the key pair.

Running the keylogin Command for Secure RPC

Normally, the login password is identical to the Secure RPC password. In this case, the keylogin command is not required. However, if the passwords are different, the users have to log in and then run the keylogin command.

The keylogin command prompts the user for a Secure RPC password. The command then uses the password to decrypt the secret key. The keylogin command then passes the decrypted secret key to the keyserver program. The keyserver is an RPC service with a local instance on every computer. The keyserver saves the decrypted secret key and waits for the user to initiate a Secure RPC transaction with a server.

If both the login password and the RPC password are the same, the login process passes the secret key to the keyserver. If the passwords are required to be different, then the user must always run the keylogin command. When the keylogin command is included in the user's environment configuration file, such as the ~/.login, ~/.cshrc, or ~/.profile file, the keylogin command runs automatically whenever the user logs in.

Generating the Conversation Key for Secure RPC

When the user initiates a transaction with a server, the following occurs:

1. The keyserver randomly generates a conversation key.

2. The kernel uses the conversation key, plus other material, to encrypt the client's timestamp.

3. The keyserver looks up the server's public key in the public key database. For more information, see the publickey(4) man page.

4. The keyserver uses the client's secret key and the server's public key to create a common key.

5. The keyserver encrypts the conversation key with the common key.

Initially Contacting the Server in Secure RPC

The transmission, which includes the encrypted timestamp and the encrypted conversation key, is then sent to the server. The transmission includes a credential and a verifier. The credential contains three components:

• The client's network name

• The conversation key, which is encrypted with the common key

• A “window,” which is encrypted with the conversation key

The window is the difference in time that the client says should be allowed between the server's clock and the client's timestamp. If the difference between the server's clock and the timestamp is greater than the window, the server rejects the client's request. Under normal circumstances, this rejection does not happen, because the client first synchronizes with the server before starting the RPC session.

The client's verifier contains the following:

• The encrypted timestamp

• An encrypted verifier of the specified window, which is decremented by 1

The window verifier is needed in case somebody wants to impersonate a user. The impersonator can write a program that, instead of filling in the encrypted fields of the credential and verifier, just inserts random bits. The server decrypts the conversation key into some random key. The server then uses the key to try to decrypt the window and the timestamp. The result is random numbers. After a few thousand trials, however, the random window/timestamp pair is likely to pass the authentication system. The window verifier lessens the chance that a fake credential could be authenticated.

Decrypting the Conversation Key in Secure RPC

When the server receives the transmission from the client, the following occurs:

1. The keyserver that is local to the server looks up the client's public key in the public key database.

2. The keyserver uses the client's public key and the server's secret key to deduce the common key. The common key is the same common key that is computed by the client. Only the server and the client can calculate the common key because the calculation requires knowing one of the secret keys.

3. The kernel uses the common key to decrypt the conversation key.

4. The kernel calls the keyserver to decrypt the client's timestamp with the decrypted conversation key.

Storing Information on the Server in Secure RPC

After the server decrypts the client's timestamp, the server stores four items of information in a credential table:

• The client's computer name

• The conversation key

• The window

• The client's timestamp

The server stores the first three items for future use. The server stores the client's timestamp to protect against replays. The server accepts only timestamps that are chronologically greater than the last timestamp seen. As a result, any replayed transactions are guaranteed to be rejected.

Implicit in these transactions is the name of the caller, who must be authenticated in some manner. The keyserver cannot use DES authentication to authenticate the caller because the use of DES by the keyserver would create a deadlock. To avoid a deadlock, the keyserver stores the secret keys by user ID (UID) and grants requests only to local root processes.

Returning the Verifier to the Client in Secure RPC

The server returns a verifier to the client, which includes the following:

• The index ID, which the server records in its credential cache

• The client's timestamp minus 1, which is encrypted by the conversation key

The reason for subtracting 1 from the client's timestamp is to ensure that the timestamp is out of date. An out-of-date timestamp cannot be reused as a client verifier.

Authenticating the Server in Secure RPC

The client receives the verifier and authenticates the server. The client knows that only the server could have sent the verifier because only the server knows what timestamp the client sent.

Handling Transactions in Secure RPC

With every transaction after the first transaction, the client returns the index ID to the server in its next transaction. The client also sends another encrypted timestamp. The server sends back the client's timestamp minus 1, which is encrypted by the conversation key.

Administering Secure RPC (Task Map)

The following task map points to procedures that configure Secure RPC for NIS, NIS+, and NFS.

Administering Authentication With Secure RPC

By requiring authentication for use of mounted NFS file systems, you increase the security of your network.

How to Restart the Secure RPC Keyserver

1. Assume the Primary Administrator role, or become superuser.

   The Primary Administrator role includes the Primary Administrator profile. To create the role and assign the role to a user, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

2. Verify that the keyserv daemon is running.

   # svcs \*keyserv\* STATE STIME FMRI disabled Dec_14 svc:/network/rpc/keyserv

3. Enable the keyserver service if the service is not online.

   # svcadm enable network/rpc/keyserv

How to Set Up a Diffie-Hellman Key for an NIS+ Host

This procedure should be done on every host in the NIS+ domain. After root has run the keylogin command, the server has GSS-API acceptor credentials for mech_dh and the client has GSS-API initiator credentials.

For a detailed description of NIS+ security, see System Administration Guide: Naming and Directory Services (NIS+).

1. Assume the Primary Administrator role, or become superuser.

   The Primary Administrator role includes the Primary Administrator profile. To create the role and assign the role to a user, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

2. Enable the publickey table in the name service.

   Add the following line to the /etc/nsswitch.conf file:

   publickey: nisplus

3. Initialize the NIS+ client.

   # nisinit -cH hostname

   where hostname is the name of a trusted NIS+ server that contains an entry in its tables for the client system.

4. Add the client to the cred table.

   Type the following commands:

   # nisaddcred local # nisaddcred des

5. Verify the setup by using the keylogin command.

   If you are prompted for a password, the procedure has succeeded.

   # keylogin Password:

Example 16–1 Setting Up a New Key for root on an NIS+ Client

The following example uses the host pluto to set up earth as an NIS+ client. You can ignore the warnings. The keylogin command is accepted, verifying that earth is correctly set up as a secure NIS+ client.

# nisinit -cH pluto
NIS Server/Client setup utility.
This system is in the example.com. directory.
Setting up NIS+ client ...
All done.
# nisaddcred local
# nisaddcred des 
DES principal name : unix.earth@example.com
Adding new key for unix.earth@example.com (earth.example.com.)
Network password:<Type password>
Warning, password differs from login password.
Retype password: <Retype password>
# keylogin
Password:        <Type password>
#

How to Set Up a Diffie-Hellman Key for an NIS+ User

This procedure should be done on every user in the NIS+ domain.

1. Assume the Primary Administrator role, or become superuser.

   The Primary Administrator role includes the Primary Administrator profile. To create the role and assign the role to a user, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

2. Add the user to the cred table on the root master server.

   Type the following command:

   # nisaddcred -p unix.UID@domain-name -P username.domain-name. des

   Note that, in this case, the username.domain-name must end with a dot (.).

3. Verify the setup by logging in as the client and typing the keylogin command.

Example 16–2 Setting Up a New Key for an NIS+ User

In the following example, a key for Diffie-Hellman authentication is given to the user jdoe.

# nisaddcred -p unix.1234@example.com -P jdoe.example.com. des
DES principal name : unix.1234@example.com
Adding new key for unix.1234@example.com (jdoe.example.com.)
Password:       <Type password>
Retype password:<Retype password>
# rlogin rootmaster -l jdoe
% keylogin
Password:       <Type password>
%

How to Set Up a Diffie-Hellman Key for an NIS Host

This procedure should be done on every host in the NIS domain.

1. Assume the Primary Administrator role, or become superuser.

   The Primary Administrator role includes the Primary Administrator profile. To create the role and assign the role to a user, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

2. Enable the publickey map in the name service.

   Add the following line to the /etc/nsswitch.conf file:

   publickey: nis

3. Create a new key pair by using the newkey command.

   # newkey -h hostname

   where hostname is the name of the client.

Example 16–3 Setting Up a New Key for root on an NIS Client

In the following example, earth is set up as a secure NIS client.

# newkey -h earth
Adding new key for unix.earth@example.com
New Password:   <Type password>
Retype password:<Retype password>
Please wait for the database to get updated...
Your new key has been successfully stored away.
#

How to Set Up a Diffie-Hellman Key for an NIS User

This procedure should be done for every user in the NIS domain.

Before You Begin

Only system administrators, when logged in to the NIS master server, can generate a new key for a user.

1. Assume the Primary Administrator role, or become superuser.

   The Primary Administrator role includes the Primary Administrator profile. To create the role and assign the role to a user, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

2. Create a new key for a user.

   # newkey -u username

   where username is the name of the user. The system prompts for a password. You can type a generic password. The private key is stored in an encrypted form by using the generic password.

3. Tell the user to log in and type the chkey -p command.

   This command allows users to re-encrypt their private keys with a password known only to the user.

   ---

   Note –

   The chkey command can be used to create a new key pair for a user.

   ---

Example 16–4 Setting Up and Encrypting a New User Key in NIS

In this example, superuser sets up the key.

# newkey -u jdoe
Adding new key for unix.12345@example.com
New Password:   <Type password>
Retype password:<Retype password>
Please wait for the database to get updated...
Your new key has been successfully stored away.
#

Then the user jdoe re-encrypts the key with a private password.

% chkey -p
Updating nis publickey database.
Reencrypting key for unix.12345@example.com
Please enter the Secure-RPC password for jdoe:<Type password>
Please enter the login password for jdoe:     <Type password>
Sending key change request to centralexample...

How to Share NFS Files With Diffie-Hellman Authentication

This procedure protects shared file systems on an NFS server by requiring authentication for access.

Before You Begin

Diffie-Hellman public key authentication must be enabled on the network. To enable authentication on the network, do one of the following:

• How to Set Up a Diffie-Hellman Key for an NIS+ Host

• How to Set Up a Diffie-Hellman Key for an NIS Host

1. Become superuser or assume a role that includes the System Management profile.

   The System Administrator role includes the System Management profile. To create the role and assign the role to a user, see Configuring RBAC (Task Map).

2. On the NFS server, share a file system with Diffie-Hellman authentication.

   # share -F nfs -o sec=dh /filesystem

   where filesystem is the file system that is being shared.

   The -o sec=dh option means that AUTH_DH authentication is now required to access the file system.

3. On an NFS client, mount a file system with Diffie-Hellman authentication.

   # mount -F nfs -o sec=dh server:filesystem mount-point

   server

   Is the name of the system that is sharing filesystem

   filesystem

   Is the name of the file system that is being shared, such as opt

   mount-point

   Is the name of the mount point, such as /opt

   The -o sec=dh option mounts the file system with AUTH_DH authentication.